Even yet at rare intervals, like a phantom, Hy Brasil appears far out on the Atlantic. No later than the summer of 1908 it is said to have been seen from West Ireland, just as that strange invisible island near Innishmurray, inhabited by the invisible ‘gentry’, is seen – once in seven years. And too many men of intelligence testify to having seen Hy Brasil at the same moment, when they have been together, or separated, as during the summer of 1908, for it to be explained away as an ordinary illusion of the senses. Nor can it be due to a mirage such as we know, because neither its shape nor position seems to conform to any known island or land mass. The Celtic otherworld is like that hidden realm of subjectivity lying just beyond the horizon of mortal existence, which we cannot behold when we would, save with the mystic vision of the Irish seer.[*]
A sunken realm below the horizon.
during each century with an odd number like 1900, the fairy tribes are said to be visible or to reappear among men, and to become invisible or to disappear during each century with an even number like 1800.
[W. Y. Evans-Wentz, The Fairy Faith in Celtic Countries, pp. 334 & 206]
2000 onwards is invisible